Frequency multiplier



Nov. 26, 1940. v. H. FRAENCKEL 2 3 FREQUENCY MULTIPLIER Filed D90. 19, 1939 I 2 Sheets-Sheet l Inventor" Victor H. Fr'aencKel,

y l -lis Attorney.

N v. 26. 1940 v. H. FRAENCKEL 2 222,899

FREQUENCY MULTIBLIER ANN vvv Inventor:

' Victor H. Fraenckel y His Attorney.

Patented Nov. 26, 1940 v 1 FREQUENCY MULTIPIJER Victor H. Fraenckel; Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 19, 1939, Serial No. 310,059 7 Claims. 7 (CL 250-36) The present invention relates to electronic frequency multipliers and has as an object the provision of a multiplier capable of effective operation at wave lengths on' the order of from- '6 five meters, to five centimeters or less.

. The novel features which I desire to. protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof,

10 may best be understood by reference to the following description taken in connection with the drawings in which Fig. 1 illustrates an apparatus embodying the invention, and Figs. 2, 3, 4, 5

and 6ar'e graphical representations useful in 25 density. The first type of modulation involves the production of systematic irregularities in electron velocity from point to point along the beam. The second involves the production of charge density variations, such variations being manifested as systematic irregularities in the electron grouping. In the conventional design of electronic discharge devices no distinction is made, between these two types of modulation. In connection with ultra-short-wave devices, however, it is advantageousv to utilize modulating electrodes which are capable of producing velocity modulation without simultaneously causing. appreciable charge density variations. For reasons I 40 which need not be elaborated here this expedient avoids the large input losses which are observed with conventional prior art devices when they are operated at extremely high frequencies. By'additional means, also described in the aforesaid Hahn application, velocity modulation produced as above specified may be subsequently converted into charge density modulation of a higher order of magnitude so as to produce ani- 5 plification effects.

It is found that the velocity modulation principle may be most readily utilized in a discharge device of the cathode ray type, wherein the elongated stream of electrons is susceptible of being 55 variously influenced at different points along its length. I have, therefore, chosen a device of this kind to illustrate my present invention.

Referring particularly to Fig. 1, I have shown an electron beam tube which comprises an evacuated envelope having an elongated shaft por- 6 tion Ill and an enlarged anode-containing portion ll. This envelope may be suitably .constituted of glass, quartz, or any equivalent insulating material.

The shaft portion l0 encloses means, such as a 10 known type of electron gun, for producing an electron beam. The combination shown comprises a cathode l4, which is indicated in dotted outline, and a focusing cylinder ii for confining the electrons from the cathode to a concentrated beam. This cylinder may be either connected directly to the cathode or maintained at a few volts'positive or negative with respect to it. In order to accelerate the electrons to a desired extent there is provided an accelerating electrode go It which is spaced from the cathode and which may be biased to a suitable positive potential, say several hundred volts.

In order that the intermediate portion of the beam path may be maintained at a desired potential level there are provided a number of intermediate electrodes 2! which suitably comprise rings of conducting material applied to the inner wall surface of the envelope. These are provided with external contact-making ter- 0 minals 23. A number of magnetic focusing coils 25 distributed along the envelope serve to prevent dispersion of the electrons and to maintain the beam in focus during its passage through the discharge space. In some cases these coils may be advantageously replaced by electrostatic beam-focusing means.

After traversing the envelope, the electron beam is collected by an anode I8 which consists of graphite or other suitable material. A tubular electrode IS in the nature of a suppressor grid serves to prevent secondary electrons emitted by the anode from returning to the discharge space.

In the operation of the device the intermediate electrodes 2| may be maintained at ground potential, the cathode It at one thousand to several thousand volts below ground and the anode [8 at one thousand to several thousand volts positive with respect to the cathode. The suppressor grid l9 should be biased fifty to several hundred '50 The combination of elements so far described comprises means for producing a unidirectional electron beam of substantially constant average intensity and velocity. As pointed out in the aforesaid Hahn application, Serial No. 153,602, an electron beam of this type may be velocity modulated by applying to the beam longitudinal potential gradients which vary cyclically at a desired frequency. One suitable velocity modulating structure is shown in the drawings.

This comprises a modulating chamber or space provided between the extremities of two conducting tubular members 3| and 32 which are arranged to surround the beam path. Within this space there is provided a tubular control electrode 30 which also surrounds the beam path. The tubular members 3| and 32 are shown as being grounded so that the boundaries of the modulating space may be regarded as definitely fixed. By alternately raising and lowering the potential of the electrode 30 with respect to these boundaries, variable potential gradients are produced which act longitudinally on the electron beam as it traverses the approach spaces between the electrode 30 and the extremities of the members 3| and 32. The modulating efiect thus produced will be most pronounced if the length of the tubular electrode 30 is so correlated to the velocity of the beam that the electron transit time therethrough corresponds at least approximately to a half-cycle of the control potential (or to an odd number of such half cycles). If this condition is fulfilled, an electron which enters the modulating space when the potential of the control electrode 30 is maximum is accelerated first by the gradient existing between the tube 3| and the electrode and again as it leaves the electrode a half cycle later when the electrode potential is at a mini mum with respect to the tube 32. Similarly, an electron which enters the modulating space in such time phase as to be retarded by the effect of the control electrode is also retarded as it leaves the electrode. As a result of these effects, the electron beam leaving the modulating chamber is made up of alternate elements, some of which have a velocity above the average of the beam and others a velocity below such average.

Modulating potential may be applied to the.

control electrode 30 from any desired source such as a high frequency oscillator (not shown). As a means for connecting this potential to the control electrode structure there is provided a concentric conductor transmission line comprising an inner conductor 35 and an outer conductor 36 which concentrically surrounds the inner conductor.

If only weak control potentials are available, the velocity modulation produced may be relatively slight. However, it may be converted into charge density modulation of a higher order of magnitude by a mechanism now to be described.

It will be understood that as the beam issues from the modulating space it comprises alternate groups of slow and fast electrons. This electron arrangement is suggested in Fig. 2 wherein the black dots a. represent fast electrons and the light circles b represent slow electrons. At the exit boundary of the modulating chamber the beam is still substantially uniform so far as charge density or electron grouping is concerned. At a slightly later time, as indicated in Fig. 3, the more rapidly moving electrons will catch up with the slower electrons, and electron bunches will exist from point to point along the beam. The resultant succession of charge density maxima and minima corresponds to charge density modulation as hereinbefore defined. The conversion of velocity modulation, as

produced by the electrode Ill, into charge density modulation is a matter which in its very nature requires only the elapse of time and the absence of extraneous influence which might tend adversely to aflect conditions within the beam. These requirements may be fulfilled by the provision of an electrostatically shielded drift space in which sorting of the electrons can take place. This drift space may comprise, for example, the section of the discharge envelope which is enclosed within the conducting tube 32. If this tube is made of proper length a relatively slight degree of velocity modulation may be converted into a much higher order of charge density modulation so that an amplification effect is obtained.

As long as the initial control voltage or signal is small, the charge density modulation developed in the drift space will have a pattern corresponding substantially to that of the initial velocity modulation; that is to say, the amplification will be of linear character so that no distortion is produced. While this is obviously desirable in a device intended solely for amplification purposes, it does not fulfill the requirements of frequency multiplication. In order that the latter phenomenon may occur the sys-. tem must manifest at least some non-linearity of response.

relatively great intensity so as to produce harmonic distortion in the resulting charge density modulation. The simplest way of accomplishing this would obviously be to apply a powerful modulating voltage to the electrode ll. However, since such voltages are rarely available atthe high frequencies here underconsideration it is desirable to construct the frequency multiplier itself as a multistage amplifier. In the embodiment shown in Fig. 1 the means for producing multistage amplification comprises a modulation intensifying chamber or space formed between the opposed extremities of the tube 32 and over another conducting tube 38. within this chamber there is provided a tubular electrode 40 generally similar to the electrode 30 which has already been described.

It will be readily understood that the charge density variations in the charge density modulated beam traversing the approach spaces which exist between the electrode ll and the extremities of the tubes 32 and ll will induce in the electrode cyclically varying currentsof a 'frequency corresponding to the modulation frequency. The magnitude of this induced current will be greatest if the length of the electrode 40 corresponds approximately to the spacing between adjacent charge density maxima and minima in the beam so that the approach of a charge density maximum corresponds with the recession of a charge density minimum and viceversa.

In' order that the induced current may be caused to produce the effects desired in the present connection, the electrode III and the tubes 32 and 38 (which are both at ground potential) should be connected through a high impedance circuit. In the arrangement illusaaaapaa 3 I trated such a circuit is provided by the use of a resonant transmission line' comprising an inner conductor 42 and a concentric outer conductor 48. The inner and outer conductors are directly connected to one another at one end so that the point of connection is approximately a quarter wave length from the open circuited "end of the transmission line; that is to say, the end to which'the electrode '48 is connected.

with an arrangement such as that indicated, the current induced in' the, electrode 48 will produce sustained oscillation of the transmission line and will cause a voltage maximum or antinode to exist between the electrode and the adl5 jacent extremities of the tubes 82 and 88. This voltage will be of cyclically varying character and will have a frequency determined by the. rate of approach and recession of charge density maxima in the beam; that is to say, by the frequency of the initial velocity modulating potential.

By analogy with the operation of the electrode 38, it will be seen that the potential gradients producedrby the electrode 48 necessarily act to 5 cause additional velocity-modulation of the electrode beam. Since the voltage swing ofthe electrode 48 may be very much greater than that of the input electrode 38, the magnitude of the new velocity modulation will be correspondingly *0 larger than that of the initial modulation.

. As a result of the operation described in the foregoing, the beam issuing from the electrode 48'is highly velocity modulated according to a modulation pattern determined by that of the signal voltage impressed on the input electrode 38. In accordance with the principles previously explained, this velocity modulation may be converted into charge density modulation within the conductive space 38. However, as a result of the greater intensity of the velocity modulation produced by the electrode 48, a marked distortion of wave form may be observed in the charge density modulation of the beam as it issues from the tube 38. The difference referred to may best be understood by a consideration and comparison of the graphical representations of Figs.

4, 5 and 6.

The curve A of Fig. 4 may be taken to represent both the control voltage (of fundamental frequency) assumed to be applied to the electrode 38 and the'velocity modulationassumed to be produced by such voltage. It will be noted that this quantity is of purely sinusoidal form. The curve B of Fig. 5 illustrates the charge I density modulation which appears at the far end of the drift space 32 as a result of the velocity modulation impressed on the beam by the electrode 38. It will be noted that while this mod-v ulation is greater in magnitude than the velocity 50 modulation it nevertheless resembles it 'in wave form. That is to'say, the charge density modulation of that portion of the beam entering the electrode 48 corresponds in frequency to the fundamental frequency of the original input signal. 5 At the exit end of the drift tube 38, however, a different condition prevails, as'is indicated in Fig. 6. It will be noted that curve C of this figure, which represents the wave shape of the charge density modulation of the beam issuing from the tube 38, is of distorted form. This distortion is a consequence of the high degree of velocity modulation impressed on the beam by the action of the electrode 48. As a result of the extreme acceleration of certain electrons and the correspondingly extreme deceleration of other electrons, caused by this electrodaan electron grouping more complex than that corresponding to purely sinusoidal modulation may be developed. within the tube 88. The pattern of such srouping is determined in part by the magnitude 5 9 of the velocity modulation caused by the electrode 48 and in' part by the length of the tube 88 and may be controlled to a certain extent by varying these factors. (The first factor may be fixed by adjusting the tuning of the transmission 10 line 4248 and the second factor by modifying the physical dimensions of the tube 88.) In the practical use of the device as a frequency multivelocity modulation impressed on the beam. 25

In order that the selected harmonic may predominate in the output 'of the apparatus, means may be provided for selectively amplifying such component before the beam approaches the output element of the tube l8. Such means may comprise, for example, aself-contained amplification stage generally similar to that associated with the electrode 48. In the present case, this is illustrated as including an electrode 48 which is placed between the opposed extremities of the 35 tube 88 and of another conducting tube 48. This electrode is connected to a circuit means which is selective to the desired harmonic component and which may consist, for example, of aquarter wave transmission line including concentric 4o conductors 58 and 5|. If the component intended to be amplified is a second harmonic of the frequency applied to the electrode 88, the length of the conductors 58, 5| should be approximately half that of the conductors 42 and 48 5 n to cause the proper resonance to obtain.

In accordance with the principles explained in connection with electrode 48 the reaction of the electrode 46 on the beam will be such as to cause a further velocity modulation thereof. In 50 this case, however, the modulation will vbe at a frequency corresponding to that to which the transmission line 58, 5| is tuned. It may be converted into charge density modulation by the provision of a further drift space contained with- 55 in the tube 48 which, for proper operation at the second harmonic, should be of approximately half the length of the tubes 32 and 38.

Output power is taken from the beam by means of a fourth tubular electrode 54. plained in connection with electrode 48, the action of the modulated beam in traversing the electrode 54 is to induce in that electrode currents which correspond to the charge density modulation of the beam. In order that'this ef- 5 feet may be at least partially selective as to the particular harmonic desired to be derived from the system the axial length of the electrode 54 should be approximately equal to the spacingbetween adjacent charge density maxima which oc- 7 our in the beam at this frequency. For usewith the second harmonic, this length should be ap-' proximately half that of the electrode 48. The current variations induced in the electrode 54 may be effectively utilized by connecting the I As ex- 0 electrode to a load circuit. As a coupling medium one may employ, for example, concentric conductors 58 and 51. 7

While I have, described my invention primarily in connection" with a frequency doubling system it will be understood that it may be used with equal emcacy in order to obtain selective amphflcation of other harmonic frequencies. In this connection it is only required that the various electrodesof the drift tube elements be so dimensioned as to operate effectively at such frequency.

Furthermore, while I have described the invention in connection with a particular embodiment thereof, numerous modifications may be made by those skilled in the art without departingfrom the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States:

1. In combination, means including an electron source for producing a beam of electrons, means acting on the beam at a point relatively near the electron source toproduce velocity variations in the beam at a fundamental frequency, circuit means coupled to the beam at a point relatively remote from the beam source, said circuit means being resonant at a frequency corresponding to a desired harmonic of the said funda mental frequency and being effective when excited at such harmonic frequency to produce corresponding velocity variations in the beam, and

means for abstracting energy from the beam at the said harmonic frequency.

2. A frequency multiplier comprising means for producing a. beam of electrons,-means for producing velocity modulation of the beam at a fundamental frequency, means providing a fieldfree space of substantial length to be traversed by the beam after modulation thereof, resonant circuit means adapted to oscillate at a frequency corresponding to' a desired harmonic of the said fundamental frequency and coupled to the beam at its point of issuance from the said field-free space, the said-circuit means being capable of mutual reaction with the beam so as concurrently to be excited to oscillation thereby and to cause a secondary modulation of the beam at the said harmonic frequency, and output means for abstracting: energy from the beam at the said harmonic frequency.

3. A frequency multiplier comprising means ineluding a discharge device of the cathode ray type for producing a beam of electrons, means for producing initial modulation of the beam at a fundamental frequency, an electrode structure coupled to the modulated beam at a point along the beam path, the dimensions of the electrode structure being so correlated to the electron transit time in the beam as to assure effective mutual reaction therewith, resonant circuit means adapted to oscillate at a frequency corresponding to a desired harmonic of the fundamental frequency, the said circuit means being connected to-the electrode structure so as to be excited to oscillation by the action of the modulated beam thereon, and being effective when so excited to produce secondary modulation of the beam at the saiddesired harmonic frequency, and output means for abstracting energy from the beam at the harmonic frequency.

assasoo [the beam results in a secondary modulation of the beam at the frequency of oscillation of the line, and means coupled to the doubly modulated beam for abstracting energy therefrom at the said harmonic frequency.

5. A frequency multiplier comprising a dischar e device of the cathode ray type for producing an electron beam, means for producing initial velocity modulation of the beam at a fundamental frequency, a first circuit means resonant at the said fundamental frequency, and reacting with the beam to increase the degree of modulation thereof, a second circuit means, resonant at a desired harmonic of the fundamental frequency and coupled to a portion of the beam previously affected by the first circuit means, the said second circuit means serving by mutual reaction with the beam to selectively augment the components of modulation corresponding to the said harmonic frequency, and an output system coupled to a portion of the beam previously affected by the said second circuit means for abstracting energy from the beam at the said harmonic frequency.

6. A frequency multiplier comprising adischarge device of the cathode ray type for producing a beam of electrons, means for producing initial velocity modulation of the beam at a fundamental frequency, a first'circuit means resonant at the said fundamental frequency and mutually reacting with the beam to intensify the said initial velocity modulation thereof, means providing a drift space to be traversed by the beam after' reaction with the said circuit means, the said" space being of suilicient length to permit eifective conversion of' the said velocity modulation into charge density modulation, a second circuit means resonant at a desired harmonic of the fundamental frequency and coupled to the beam at the point of its issuance from the drift space, the said second circuit means being effective by virtue of its mutual reaction with the charge density modulated beam to produce a secondary velocity modulation thereof at the said harmonic frequency, and an output system coupled to a portion of the beam previously affected by the second circuit means for abstracting energy from the beam at the harmonic frequency.

'7. A frequency multiplier comprising means for producing a beam of electrons, means for producing velocity modulation of the beam at a fundamental frequency, means providing a fieldfree space of substantial length to be traversed by the beam after modulation thereof, means resonant to a harmonic of said fundamental frequency coupled to said beam at the terminus .of said space, said means reacting to'cause secondary modulation of said beam at said harmonic frequency, and means for abstracting energy from said beam at said harmonic frequency.

VICTOR H. FRAENCKEL. 

